1. Field of the Invention
The present invention relates to a wideband radio receiver and a radio receiving method for receiving radio signals.
2. Related Art
With development of informationalized society, communication means that can be used in close-at-home scenes increase by leaps and bounds and the increase in communication rate is also remarkable. There are instances in which bands that have been used for a specific use are diverted to a different use in order to increase the communication rate. For example, in radio communication, the law stipulates that a specific band should be limited to a specific use; however, some bands are opened to a plurality of uses. In future, there is a possibility that a very wide band such as UWB (Ultra-wideband) will be opened only to low-power-short-distance-communication so as to overlap other uses. In addition, study of a scheme called cognitive radio has been started. In the cognitive radio, an unlicensed terminal can use a band a specific system is licensed to use under present conditions, only in a place where and at time when the specific system does not use the band (see Japanese Patent No. 3583962).
The basic concept of the cognitive radio is to scan a band having the possibility of use, detect the state of use, search for a usable band and use it. Taking higher rates and wider bands in radio communication in recent years into consideration, it is expected to scan a very wide band and determine to use bands that are included in the very wide band and that are capable of satisfying required performance.
In addition, diversification of the connection form has advanced. Hitherto, the destination to which a radio terminal is connected has been substantially one party, such as a base station, at one time. However, it is expected that many-to-many connections will increase from now on and it will become necessary to connect to a plurality of destinations at the same time.
In the cognitive radio, idleness in the licensed system is detected and the gap is used, as described above. As compared with the bandwidth of the radio signal itself, therefore, the frequency band in which a terminal has capability for transmission and reception is very wide. In addition, since the frequencies in use change according to the states of use of licensed systems, the center frequency cannot be determined at the time of the device design. In other words, no matter which center frequency the propagated radio signal has, it must be received. In addition, with the diversification of the connection forms, there is a possibility that it becomes necessary to receive a plurality of signals that are not predetermined in frequency, at the same time. When it is attempted to receive a plurality of wideband signals respectively having arbitrary frequencies in a very wide band, such as a band between 3 GHz and 10 GHz, and bandwidths of several tens to 100 MHz, several problems are caused.
In the traditional radio communication, signals are typically present in one definite frequency band. Even if a function of receiving a plurality of signals at the same time is demanded as in a receiver in a base station, center frequencies and bandwidths of all signals are previously already known at the stage of design and the relative bandwidth of the whole bandwidth that should contain those signals is comparatively small. Therefore, it is sufficient to prepare as many receivers associated with respective frequencies or as many receivers that can be tuned in a comparatively narrow range as the number of supposed simultaneous connections.
On the other hand, in the cognitive radio, a terminal might be necessary to receive seven signals dispersed around 4 GHz in frequency in a certain time zone and receive nine signals dispersed around 9 GHz in frequency in another time zone. According to analogical inference from an ordinary radio receiver, in such a case, for example, ten receivers that can be tuned in the range between 3 GHz and 10 GHz are provided. However, the tuning range is too wide and it cannot be said to be realistic. If it is designed to have ten receivers, an eleventh signal cannot be received, resulting in limited performance. Yet, it is not realistic to previously provide, for example, 100 receivers.